Part of the description of Sam on page 11 reads:
In addition, Sam's strength is 5 when in the same region as Frodo. The Fellowship must reveal Frodo (before cards are chosen) in order to prove Sam's strength. Alternatively, the Fellowship may choose to not reveal Frodo and keep Sam's strength at 2.
Question: Under what circumstances would you choose to not reveal Frodo? Apart from the Shire and Mordor, most of the areas on the map can only hold two characters at a time. So if you want to substitute Sam in Frodo's place it is pretty obvious who the other character is.